The present invention relates to an improved design of projection for use in electrical resistance welding of sheet metal and thin-wall parts, a method of fabricating such projection and to the use of such projection.
During the mass production of parts by resistance welding it is projection welding of ferrous metals which constitutes one of the most widespread used welding techniques for obtaining high quality connections. In contrast to spot welding during projection welding the expanse or extent of the weld connection is not governed by the cross-sectional area of the electrode, rather by the cross-sectional area of the projection. On the one hand, there is thus exactly defined and maintained constant the welding surface and therefore the current path, the current density and the surface compression and, on the other hand, there also exists the possibility of simultaneously welding a multiplicity of projections and under exactly the same conditions and of preventing current shunts.
In the case of non-ferrous metals, especially, aluminium and aluminium alloys, it is not possible to positively perform the known projection welding when working with thin sheets. This is predicated upon the fact that aluminium, in contrast to iron, opposes the welding current with considerably less electrical resistance and at the same time appreciably better conducts the thermal energy which is produced by the current, and hence, conducts such away from the site which is to be welded. A further difficulty in addition thereto is that upon application of the force needed for welding at the workpiece, i.e. the projection, it is necessary for such to be larger than in the case of iron and such projection collapses before the current can be turned-on. If there is used a smaller electrode force then the projections can score or burn before there arises the actual welding operation because at the contact region between the projection and sheet metal there is present too great resistance.
Therefore it has already been proposed to undertake measures wherein the electrode is applied extremely gently at the workpiece and by means of an adjustable timing mechanism the complete electrode force is first then applied directly prior to the presence of the welding current.
Also in the case of machines working with program controls for the mutual timewise coordination of the course of the current and pressure the heretofore known methods have not up to now enabled any satisfactory randomly reproducible projection welding for the mass production of parts.
Equally, in the pertinent more recent publications, for instance "Resistance Welding Manual", Volume 1, page 44 and Aluminium-Taschenbuch 13, Aufl. 1974, page 581, there is repeatedly mentioned that projection welding of aluminium can not be positively carried out. Significant in this respect is also the work of S. A. Westgate and R. M. Rivett, entitled "Effect of Projection Design when Welding Single Projections in Aluminium Autobody Sheet", March 1979, 89/1979, of the Welding Institute. Nonetheless while a further publication authored by Pfeifer, entitled "Fachkunde des Widerstandsschweissens", states broadly at page 44 thereof that projection welding of light metals is possible "with suitable machines", yet there is not given any specific teaching as to how such type welding is capable of being performed.
From the prior art publications and from practice it is known to weld aluminium with embossed projections and this technique also has been found to be useful. Embossed projections only can be provided at solid parts, for instance handle projections or extensions of pans; at sheet metal parts or other thin-wall parts it is totally impossible to economically produce solid projections.